Treatment of contaminated gases

ABSTRACT

A contaminant is removed from a gas in a method in which the gas is contacted with a solvent for the contaminant, and the resultant solution is contacted with intercalated graphite onto which the contaminant is adsorbed. The solvent will normally comprise water, and may itself be a contaminated liquid. In a preferred method the solvent is in a reservoir with particulate intercalated graphite; the contaminated gas is delivered to the base of the reservoir; and the gas rises in the solvent mobilizing the intercalated graphite in the reservoir.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S. patent applicationSer. No. 12/760,971, which was filed Apr. 15, 2010, which is acontinuation of International Patent Application No. PCT/GB2008/003532,which was filed Oct. 17, 2008, which claims priority to Great BritainPatent No. 0720429.0, which was filed Oct. 18, 2007. The entire text ofthe aforementioned applications is incorporated herein by reference inits entirety.

This invention relates to the treatment of fluids to removecontaminants, and particularly to the removal of contaminants from airand other gases. It has especial application in the removal of odours,but can also be useful in removing contaminants not: having an aroma.Although the invention has a primary application in the anodic oxidationof organic compounds, it can also be used for the cathodic reduction ofcompounds, and also for disinfection. For example, it can be used totreat combustible gases for the removal of compounds such as siloxanesand for the treatment of gases arising from land remediation.

Reference is directed to International Patent Publication No:WO2007/125334, which discloses apparatus for the treatment of liquid bycontact with a particulate adsorbent material. The entire disclosure ofPublication No: WO2007/125334 is incorporated herein by reference.Preferred adsorbent materials used in that apparatus are graphiteintercalation compounds (GICs). Such adsorbent materials are alsodisclosed in our British Patent Application No: WO2008/047132, alsoincorporated herein by reference.

The present invention recognises that adsorbent materials of the kinddescribed in the applications referred to above can be used directly andindirectly in the treatment of air and other gases to removecontaminants and odour containing compounds. Accordingly, in itsbroadest aspect, the present invention is a method of removing acontaminant from a gas in which the gas is contacted by an intercalatedgraphite compound. This can be done directly, or indirectly by firstcontacting the gas with a solvent for contaminant, and then contactingthe resultant solution with intercalated graphite onto which thecontaminant is adsorbed. The solvent is typically water, and it mayitself be contaminated. It may also be another liquid, contaminated orotherwise, and can be one selected for absorbing one or more particularcontaminants.

Where the contaminant is first passed to a solvent, the contaminated gasis normally passed through the solvent as bubbles to transfer thecontaminant thereto. This procedure can be incorporated in apparatus ofthe kind described in International Patent Publication No: WO2007/125334by confining the solvent in a reservoir with the intercalated graphite,and delivering the gas to the base of the reservoir such that it risesin the solvent and mobilises the intercalated graphite in the reservoir.The rising gas can thus provoke circulation of the intercalatedgraphite, and as described, the apparatus can include a zone in whichthe intercalated graphite can be regenerated during its recirculation.

It will be appreciated from the above that contaminated gas andcontaminated liquids can be treated simultaneously, provided of coursethe respective contaminants will be adsorbed by the intercalatedgraphite. The extent to which such simultaneous treatment is possiblewill depend upon the level of contaminants present, and the capacity ofthe intercalated graphite and the regeneration mechanism. It will beappreciated though, that the amount of intercalated graphite can ofcourse be selected for a particular task and the rate at which it isrecirculated can of course be controlled as can the residence time ofthe solvent in the reservoir.

In another variation, the same solvent can effectively be used twice inthe process of removing contaminants from a gas. The gas can firstcontact the solvent for direct transfer of the contaminant thereto, andthen be recycled through the solvent with the intercalated graphitewhereby further transfer of contaminant from the gas takes place, withthe contaminant being eventually adsorbed by the intercalated graphite.This initial treatment can be conducted for example in a scrubbing towerprior to recycling of the gas and liquid to a treatment reservoir of thekind referred to above.

An embodiment of the invention will now be described by way of exampleand with reference to the accompanying schematic drawings wherein:

FIG. 1 is a vertical cross-section through apparatus for conducting amethod according to the invention;

FIG. 2 is a horizontal cross-section taken on line A-A of FIG. 1; and

FIG. 3 illustrates how auxiliary treatments can be coupled to theapparatus of FIG. 1.

The drawings show a reservoir 2 of generally rectangular cross-sectiondefined by front and rear walls 4 and 6, and side walls 8. Within thereservoir, inner walls 10 define a regeneration chamber that extends thewhole width of the reservoir between the front and rear walls 4 and 6(FIG. 2). The base of the regeneration chamber is defined by convergentwalls 12, which form an opening 14 for the discharge of particulateadsorbent material from the regeneration chamber. Upper walls 18 definea central zone over the regeneration chamber.

When the apparatus is ready for use, an adsorbent material is loadedinto the regeneration chamber 16 in the required amount. Liquid is thendelivered to the reservoir through inlets 20, and filled to a level justbelow that of the discharge outlet 22 between the upper walls 18.Contaminated gas or air under pressure is then delivered throughopenings in the base of the reservoir as indicated at 24. This generatesbubbles in the liquid, and draws particulate adsorbent material frombelow the opening 14 at the bottom of the regeneration chamber, andcarries it upward through treatment chambers 26 defined in the reservoirbetween the respective walls 8 and 10. The rising air or gas carries theadsorbent material around and over the top of the walls 10, where it isguided by the walls 18 back into the regeneration chamber. Obstacles 28and 30 are installed at the top of the regeneration chamber to controlthe flow of the solid, liquid and gaseous phases in the reservoir. Theycan break up any coagulated particles and guide them into the chamber.They also serve to discourage adsorbent particles from entering the zonebetween the walls 18, from which liquid can overflow, and preventbubbles generated in the bed of adsorbent materials in the regenerationchamber from entering this zone.

As the contaminated air or gas rises in the reservoir 2 the contaminantis absorbed by the liquid. As the liquid and adsorbent materialcirculate in the reservoir, transfer of the contaminant continues fromboth the gas and liquid to the adsorbent particles which can becontinuously regenerated in the regeneration chamber.

If the adsorbent material is being used to decontaminate both the liquidand the gas, then provision can be made for both the liquid and the gasto be flowing through the reservoir. In this variant, the liquid isdelivered to the reservoir through the inlets 20 at a flow rate selectedto match its required residence time in the reservoir and contact withthe adsorbent material sufficient to enable absorption of contaminantstherefrom. Its general flow is upwards through the reservoir, and it isdischarged by overflow through the port 22. It will be noted that theliquid can only reach the discharge port 22 by upward flow from the topof the regeneration chamber, between the walls 18. The walls 18 thusdefine a quiescent zone protected from movement generated by the airbubbling through the liquid in the treatment chambers.

As bubbles of gas rise in a liquid, particularly a liquid in which solidparticles are entrained as in the present invention, they will tend tocoagulate into larger bubbles, and consequently present less surface perunit volume of gas for absorbance of the contaminant(s) by the liquid.To counter this, a stirrer or agitator 44 can be installed at anintermediate level in the reservoir 2 to break up enlarged bubbles.

If an additional treatment is used to transfer contaminant from the gasto the liquid, this can be conducted in one or more scrubbing towers 46separate from the main reservoir 2, as shown on the left in FIG. 3. Thiscan provide an initial treatment with gas entering at openings 48 andleaving at 50. Liquid enters at 54 and leaves at 56. The treated gasthen being delivered along pipe 52 to the openings 24 in the mainreservoir 2 and the liquid to inlets 20. Alternatively as shown on theright in FIG. 3, liquid, with some entrained gas bubbles and adsorbentparticles can be withdrawn from an intermediate point 58 in thetreatment chambers 26, to one or more scrubbing towers 60, andcontaminated gas fed to the towers at openings 62. The gas is thenpassed via outlet 64 and pipe 66 to the openings 24 in the mainreservoir 2, and the liquid returned to the main treatment chambers 26at a point 68 downstream of (above as shown) the point 58 or if desired,at the lower inlets 20. The use of one or more scrubbing towers in thisalternative way effectively increases the residence time for the gasbubbles in the liquid in the apparatus of FIG. 1, but without increasingthe height of the regeneration chamber defined by the walls 10.

Although illustrated as coupled to the same reservoir 2, it will beappreciated that the two types of scrubbers will not normally be used inthis way. Typically one or other auxiliary treatment will be used; notboth.

While a generally upward flow of liquid to be treated in reservoir 2 ispreferred, the opposite arrangement can also be used. Thus, liquid to betreated could be admitted at ports indicated at 32, and withdrawn fromdischarge points 34. Some form of filter would be required at thedischarge points because of the proximity of the adsorbent material, butthe air flowing upwards from the reservoir base should preventblockages. The direction of flow of liquid through the reservoir will ofcourse be selected on the basis of the system requirements, but theremay be some benefit in having the flow of liquid generally opposite tothe flow of adsorbent material in the treatment chambers. That would bethe case if the general direction of flow of liquid in the reservoir wasdownwards rather than upwards.

As noted above, the apparatus may be used for the separate treatment ofindividual volumes of liquid. In this variant, the reservoir is filledwith liquid to the required level, and the adsorbent material recycledthrough the regeneration chamber for a period of time appropriate tocomplete the treatment. The liquid is then removed, for example bydrainage from discharge port 34, and a fresh charge of liquid deliveredto the reservoir. The adsorbent material will normally be regeneratedwhile it is recycled during the treatment process.

In apparatus of the invention, the adsorbent material is continuously orintermittently regenerated while it passes through the regenerationchamber in its recycling path. Pollutants are released by theregenerating adsorbent material in gaseous form, from the top of thereservoir. These released gases can be discharged to the atmosphere, butcan of course be subject to separate treatment if required. They couldif needed, be separated from the contaminated gases by redesigning theupper walls 18 to prevent the bubbles from the adsorption (26) andregeneration (16) zones combining. They must of course be separated fromthe decontaminated gases, and to accomplish this valved seals areprovided between the inner walls 10 and the upper walls 18 to permit thedownward flow of liquid and adsorbent material, but prevent the upwardflow of gases released from the regenerated adsorbent material.

Regeneration of the adsorbent material is accomplished by theapplication of an electrical voltage between an anode 36 and a cathode38 disposed on opposite faces of the chamber 16. The cathode is housedin a separate compartment 42 defined by a conductive/permeable membrane40. This enables a catholyte to be pumped through the compartment, andthe membrane protects the cathode from direct contact with the adsorbentmaterial.

By way of example, fresh GIC particles will absorb up to 8 mg/g ofethane thiol and 7 mg/g of methyl propane thiol. A regenerationefficiency of 100% can be achieved with a charge of 30 Coulombs per gramof GIC. In a particular example, passing a current of 0.5 Amps through20 g of GIC particles for 20 minutes will result in the fullelectrochemical regeneration of its adsorptive capacity. The presence ofsodium sulphite in the solvent liquid, at concentrations of between 125and 200 mg/l, have been shown to give a significant increase in theabsorptive capacity of GIC for both ethane thiol and methyl propanethiol.

The purpose of the membrane 40 is to prevent the solid adsorbentparticles coming into contact with the cathode 38 as this could resultin the electrons going direct from cathode 38 to anode 36 withoutpassing through the aqueous phase. In this case there would be noorganic oxidation and no regeneration of the adsorbent. The membrane 40must allow the transfer of ions or electrons through it to complete theelectric circuit. However, this introduces an additional resistance intothe system. Such membranes also only operate well at certain pH levels.In this case the oxidation of the water on the anode side (giving acidconditions) and reduction of water on the cathode side (giving alkaliconditions) necessitates pH adjustment to keep the membrane functioningwith an acceptable voltage. In practice this requires the catholyte tobe monitored and adjusted to keep it acidic, for example by the constantaddition of acid, which is undesirable, the pumping of catholyte throughthe cathode compartments, and suitable pH monitoring and adjustmentequipment involving tanks, pumps and probes, which incurs furthercapital, operational and maintenance costs.

An alternative to the use of a conductive membrane is to use a porousfilter. This would prevent the contact of the solid with the cathode,but allow the passage of water and ions. The constant reduction of waterat the cathode would result in the catholyte becoming more alkaline,giving a higher conductivity and lower cell voltages. Transfer of thehydroxide ions across the porous filter would neutralise the hydrogenions generated in the anode compartment.

The invention claimed is:
 1. A method of removing a contaminant from agas, comprising contacting the gas with a solvent for the contaminant,and contacting the resultant solution with unexpanded intercalatedgraphite onto which the contaminant is adsorbed.
 2. A method accordingto claim 1 wherein the solvent is itself a contaminated liquid.
 3. Amethod according to claim 1 wherein the solvent comprises water.
 4. Amethod according to claim 1 wherein the gas is passed through thesolvent to transfer the contaminant thereto.
 5. A method according toclaim 4 wherein the solvent is in a reservoir with the intercalatedgraphite, and the contaminated gas is delivered to the base of thereservoir such that it rises in the solvent and mobilises theintercalated graphite therein.
 6. A method according to claim 5 whereinthe rising gas provokes circulation of the intercalated graphite in thereservoir.
 7. A method according to claim 5 including the step of firstcontacting the gas with solvent for direct transfer of the contaminantthereto prior to delivering the solvent to the reservoir.
 8. A methodaccording to claim 7 wherein after said contact with the solvent, thesame gas is delivered to the base of the reservoir for furtherdecontamination.
 9. A method according to claim 7 wherein the step ofcontacting the gas with solvent prior to delivering the solvent to thereservoir occurs in a scrubbing tower.
 10. A method according to claim 5including the step of agitating the solvent with entrained gas andintercalated graphite to break up large bubbles of gas therein.
 11. Amethod according to claim 5 wherein the reservoir has a zone in whichthe intercalated graphite can be regenerated.
 12. A method according toclaim 1 wherein the intercalated graphite is in particulate form.